The invention relates to a method of transferring, via a transmission medium comprising a plurality of virtual, asynchronously time-division-multiplexed transmission channels, a data stream fed to said transmission medium and composed of dam cells which are each transferred via one of said transmission channels and which each contain a channel designation indicating the transmission channel via which said dam cell has to be transferred, in which method a monitoring apparatus at the beginning of the transmission medium stores, for each transmission channel, a variable value which, when a data cell arrives at said monitoring apparatus, is amended as a function of the time which has elapsed from a reference time instant known to the monitoring apparatus, which amended variable value is then compared with a threshold value and, depending on the amended variable value and said threshold value, access to the downstream transmission medium is granted or refused to said data cell.
Such a method is disclosed inter alia, by EP-381275 in the name of the Applicant. In the latter, to achieve a time-division-multiplexed usage of a `leaky-bucket` monitoring apparatus, the time difference between the arrival time instant of the penultimate dam cell and the arrival time instant of the last data cell to arrive from the same channel is always calculated on the arrival of a data cell. Said time difference is then multiplied by a constant `leakage` value and the result thereof is subtracted from the variable value which is stored in the memory and which will also be designated hereinafter as `counter position`. The counter position is further amended by increasing it by a constant value. The result is compared with a threshold value. If the counter position is not higher than the threshold value, the data cell is allowed through to the transmission medium. The updated counter position is stored in the memory as the new counter position. In addition to the counter position, however, the time instant of the appearance of the last data cell is also stored in order to be able to update the counter position on the appearance of a subsequent data cell from the same channel. In the known method, the reference time instant, which is intended for determining the time which has elapsed, on the basis of which the counter position is amended, is therefore always and exclusively formed by the time instant of the arrival of the penultimate data cell (allowed through); in the known method, the monitoring apparatus is therefore `passive` as regards the reference time instant.
Since in practice a large number of channels (for example, 1024) is involved, it is desirable to limit the storage of variables to a minimum, and it is to this that the present invention intends to form a contribution.